Psoas

The psoas is the only major muscle that connects the lower body to the torso. It runs from the base of the femur to the lower spine on the back side of your body, deep under layers of other muscles, including the abdominals. When it's tight, you can lose range of motion in the lower back, shoulders, and hips.

An injury to the psoas might cause twitches to your stride; pain running uphill, walking up stairs, or lifting your knees; or pain in the hips, groin, glutes, or back.